Dielectric Spectroscopy Analysis Of Structure Transition Mechanism And Interfacial Polarization Properties Of Ionic Liquid Microemulsions

Ionic liquid microemulsion combines the advantages of both microemulsion and ionic liquid,which can overcome the dissolution limitation of ionic liquid,provide environmentally friendly or nano-sized with high conductivity region,reduce the amount of ionic liquid,overcome the shortcomings of mass transfer rate decrease caused by excessive viscosity of ionic liquid,and maintain the microemulsion structure in a wide temperature range.Thus,it has become a research hotspot in the field of colloid and interfacial chemistry.At present,compared with resonance type dielectric spectroscopy,relaxation type dielectric spectroscopy is rarely used to study ionic liquid microemulsions.Dielectric relaxation spectroscopy can provide information about the transformation mechanism of percolation structure,interface polarization properties and phase parameters of the system,which is a favorable supplement to physical and chemical properties.In this paper,the dielectric spectrum is innovatively adopted to analyze four different ionic liquid microemulsions,and the dielectric properties of related ionic liquid microemulsions are comprehensively analyzed by changing the types of oil phase and surface active substances respectively.The main research results are as follows:1.A hydrophilic microemulsion consisting of ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate（[C₄mim][BF₄]）as the polar phase,triton TX-100 as the surfactant and triethylamine as the oil phase,namely[C₄mim][BF₄]/TX-100/triethylamine,the dielectric relaxation spectra were measured across three different microzones.A dielectric relaxation phenomenon was observed around the frequency of 100 MHz.The dielectric parameters of the system were obtained by fitting the permittivity and dielectric loss of the relaxation with the model function.The dependence curves of dielectric parameters on ionic liquid weight fraction show that the system has undergone two phase transitions from ionic liquid-in-oil（IL/O）microdroplets to bicontinuous structure and then to oil-in-ionic liquid（O/IL）microdroplets.The determined boundary concentrations are 10.87 wt%and 26.33 wt%,respectively;The percolation threshold was 24.36 wt%obtained by two methods,and the percolation index was 0.78 by discussing four physical quantities based on the percolation theory,indicating that the system undergoes static percolation process;According to Hanai theory,the relaxation time of interfacial polarization is between 1.86 ns and 4.54 ns,which is consistent with the dielectric relaxation time obtained by experiment,indicating that the relaxation phenomenon of the system is caused by interfacial polarization,and the phase parameters of the dispersed phase and the continuous phase are obtained.2.The second system of[C₄mim][BF₄]/TX-100/toluene ionic liquid microemulsion was obtained by using toluene instead of triethylamine as oil phase,and its dielectric properties were studied.A dielectric relaxation was observed in the frequency range of 100 MHz,which was determined to be caused by interfacial polarization by theoretical calculation.The fitting dielectric parameters provided three different micro-regions with boundary concentrations of18.25 wt%and 36.00 wt%,respectively;With the determined percolation threshold of 24.36wt%,the percolation sub-index and the total index were calculated to be 0.76 and 0.78,respectively,which was is consistent with the prediction of static percolation theory,indicating that the system changed from IL/O structure to O/IL structure after bicontinuous structure.Hanai theory was used to calculate the phase parameters,which quantitatively described the dielectric property difference between the dispersed phase and the continuous phase.3.The surfactant TX-100 in the second study system was replaced by ethanol,then the[C₄mim][BF₄]/ethanol/toluene microemulsion was obtained,and the dielectric behavior of this surfactant free ionic liquid microemulsion was studied.A dielectric relaxation occurs near the 1GHz frequency.Maxwell-Wagner theory confirms that the relaxation is due to the interfacial polarization,indicating that the amphiphilic solvent ethanol is distributed at the interface;The percolation threshold was calculated as 31.09 wt%by the permittivity and conductivity,and then combined with the permittivity and loss angle of the percolation threshold sample,the percolation sub-index and total index were calculated as 0.68 and 0.72,respectively,so that the static percolation process occurred in the system.The boundary concentrations between IL/O,bicontinuous and O/IL microregions were determined to be22.39 wt%and 31.09 wt%,respectively.The different phase parameters of dispersed phase and continuous phase were calculated by Hanai theory,which supplemented the quantitative description of the electrochemical model of ionic liquid microemulsion.4.The dielectric relaxation spectra of[C₄mim][BF₄]/1-Buthyl-3-methylimidazolium1,4-bis（2-ethylhexyl）sufosuccinate（[C₄mim][AOT]）/benzene,a microemulsion formed by ionic liquid replacing water and surfactant at the same time,have been measured.The conductivity data were divided into three different concentration ranges of IL/O,bicontinuous and O/IL microzones in the experimental path,and the boundary concentrations were 15.24wt%and 29.47 wt%respectively;The percolation threshold was determined to be 21.53wt%,and the percolation index and sub-index were 0.79 and 0.75,respectively,indicating that static percolation process occurred in the system;Two dielectric relaxations are found at low frequency 10 MHz and high frequency 300 MHz respectively.The low frequency relaxation is due to the orientation polarization of anion in[C₄mim][AOT],and the high frequency relaxation is due to the interface polarization.The high frequency relaxation dielectric parameters are introduced into Hanai theory to calculate the physical phase parameters of dispersed phase and continuous phase.